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In early 1980-s Johnston et al; describe small vesicles which are shed from reticulocytesduring reticulocyte maturation. They make observations that population of releasedvesicles carry similar characteristics as reticulocyte plasma membrane. Researchersentitle these vesicles to exosomes. Since reticulocytes loose their transferrin receptorduring maturation the researchers hypothesize that exosomes could be involved inabolishing cellular cargo (Johnstone et al; 1987). Today, exosomes are defined as round(or cup) shaped, intraluminal vesicular structures 30-100 nm in size, which are activelysecreted through exocytotic pathways, within ubiquitous membrane-bound organelle, themultivesicular body (MVB).Evidence for exosomes derivation from late endosomatic compartments first arrived withthe observations with electron microscopy and was later firmly verified with Rab 27a and27b proteins to play a role in MVB fusion with plasma membrane (Ostrowski et al., 2010).The MVB, which is the central unit in exosome biogenesis is generated through thedegradative endocytosis route, by starting with the invagination from plasma membraneand developing into early endosome (EE) organelle. EE then serves as the protein sortingcompartment form which certain proteins are recycled back to plasma membrane or toGolgi network and sorted from those that mature into MVBs and presumably enter forlysosomal degradation (Katzmann et al., 2001).Human cells embody more than 60 members of Rab proteins, that are each individuallyacquainted as the unique coordinators for vesicular trafficking between variousintracellular compartments. Because Rab proteins constitute an overall conservedstructure, the switch between the active (GTP-bound) and inactive (GDP-bound) statedetermine the molecular machineries behind these molecules. In their GTP-bound form,Rab proteins are located on the interior side of the membrane and are able to recruit orcooperate with their effector molecules to facilitate vesicular fusion (Hutagalung andNovick et al., 2011).Rab5 corresponds to the junction between the cell surface clathrin-coated pits and EE.Rab5 in its active form, operates as rate limited factor and is also considered an EEmarker. Rab5-GTP effector molecule is EEA1 (early endosome antigen 1) andphosphatidylinositol 3-phosphate (PI3P), thus by binding Rab5-GTP to EEA1 and in turnEEA1 carboxy terminal binding to PI3P via FYVE finger on another vesicle, it conductstethering between endosomes in a homotypic manner (Bucci et al., 1992; Simonsen etal.,1998).PI3P, again is a link between endosomal sorting complex required for transport (ESCRT)machinery that represents a well orchestrated system for lysosomal cargo sorting anddown regulation, where small mono-ubiquitins upon tagged proteins are recognized byspecific ESCRT sub-complexes and finalize in the architecture of MVBs. The MVBsorting is initiated by ESCRT-0 (Hrs humans) by binding PI3P to localize and cluster theubiquitin cargo on endosomal membranes, which is then followed by sequential steps ofother ESCRT sub-complexes. ESCRT-I (TSG101 humans) and its fundamental role inESCRT machinery- cargo selection and concentration. ESCRT II (Alix humans) isrecruited by ESCRT I and serves as an adopter molecule between ESCRT I and -III, itspolymerization will finally recruit ESCRT III for ub recycling prior MVB invaginationand Vps4 AAA-ATPase is responsible for final dissociation. During cargo sorting andclustering, the endosomal membrane undergo secondary invagination, giving rise to anendosome lumen with intraluminal cargos (Katzmann et al., 2001; Katzmann et al., 2003;Babst et al., 2002).

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